UNIVERSITY   OF   CALIFORNIA 

COLLEGE    OF    AGRICULTURE 
AGRICULTURAL   EXPERIMENT  STATION 

CIRCULAR  No.  230 

TESTING  MILK,  CREAM,  AND  SKIM  MILK 
FOR  BUTTERFAT 

By  J.  C.  MARQUAKDT 
Division  of  Dairy  Industry 

This  circular  describes  an  approved  and  practical  method  of  using 
the  Babcock  Test  that  is  suitable  for  the  dairyman,  the  manufacturer 
of  dairy  products,  and  the  official  test  supervisor.  The  directions 
given  below  are  expressed  in  such  a  manner  that  the  tests  can  be 
made  by  persons  not  familiar  with  chemical  analysis  or  laboratory 
practices.  However,  persons  not  familiar  with  such  procedure  would 
find  it  helpful  to  observe  the  work  being  done  at  a  local  creamery  or 
other  place  where  tests  are  being  made  before  attempting  to  operate 
the  test  independently. 

TAKING    THE    SAMPLE 

A  proper  sample  is  of  first  importance  in  any  analysis.  If  the 
sample  is  improperly  taken,  the  results  will  be  of  no  value,  regardless 
of  the  operator's  knowledge  or  skill.  If  the  milk  is  mixed  by  pouring 
from  one  container  to  another  a  proper  representative  sample  can  be 
obtained.  This  method  is  used  in  connection  with  cow-testing  work. 
The  milk  from  a  complete  milking  is  poured  into  another  pail  and 
sampled  at  once.  It  is  essential  that  the  sample  be  taken  immediately 
after  mixing,  since  the  fat  soon  begins  to  rise  again.  In  large  vats  the 
mixing  may  be  successfully  accomplished  by  the  aid  of  mechanical 
agitators. 

When  samples  of  milk  are  taken  directly  from  the  can,  a  milk 
stirrer  (Fig.  1)  may  be  used  to  thoroughly  mix  the  top  and  bottom 
milk  immediately  before  the  sample  is  secured. 

Composite  sampling. — A  composite  sample  is  composed  of  pro- 
portional quantities  of  different  lots  of  milk.  In  the  official  test  work, 
composites  are  taken  with  a  graduate  pipette  and  a  certain  quantity 
of  milk  is  taken  for  each  pound  produced  by  the  animal  at  each 
milking.     The  common  practice  is  to  take  3  cubic  centimeters  per 


Z  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

pound.  This  enables  the  superintendent  of  dairy  tests  to  check  the 
work  of  the  test  supervisor  by  comparing  the  test  of  the  composite 
sample  with  the  computed  average  test.  Composite  samples  should 
be  kept  in  a  clean,  well-sealed  container  to  prevent  the  evaporation 
of  moisture.  Approved  types  of  containers  are  shown  in  Fig.  1,  and 
bottles  with  a  capacity  of  four  to  sixteen  ounces  are  used  for  preserv- 
ing the  samples.  Those  most  commonly  used  in  milk  plants  have 
a  capacity  of  six  to  eight  ounces. 

In  order  to  reduce  the  labor  and  expense  of  testing,  the  composite 
method  of  sampling  is  practiced  by  most  large  whole-milk  plants.  The 
type  of  samplers  commonly  used  in  sampling  large  quantities  of 
milk  are  also  shown  in  Fig.  1. 

THE   BABCOCK   TEST 

The  Babcock  Test  is  based  upon  the  fact  that  strong  sulphuric 
acid  will  dissolve  the  milk  solids  other  than  fat,  and  produce  heat 
which  causes  the  fat  globules  to  come  together  more  easily.  By  cen- 
trif uging  in  specially  constructed  bottles,  the  milk  fat  is  completely 
separated  from  the  serum  which  has  a  higher  specific  gravity. 

Test  bottles. — The  bottles  used  for  testing  milk  by  the  Babcock 
method  are  marked  with  divisions  on  the  neck  which  read  from  zero 
to  8  per  cent  when  17.5  cubic  centimeters  of  milk  (18  grams)  is  used 
for  the  test.  Each  of  the  smaller  divisions  on  the  neck  of  the  bottle 
represents  one-tenth  per  cent.   (.1%). 

Milk  test  bottles  marked  with  divisions  on  the  neck  reading  from 
zero  to  10  per  cent  are  sometimes  used.  Each  of  the  smaller  divisions 
on  the  neck  of  these  bottles  represents  two-tenths  per  cent  (.2%). 
The  use  of  the  10  per  cent  bottle  is  not  legal  in  California,  and  con- 
sequently, the  8  per  cent  bottle  must  be  used  for  official  testing. 

Milk  pipettes. — The  glass  milk  pipette  shown  in  Fig.  2  is  used  for 
measuring  the  quantity  of  milk  required  for  the  test.  A  mark  sur- 
rounding the  neck  of  the  pipette  indicates  the  measure  of  milk  to 
be  used.  When  the  pipette  is  filled  with  milk  to  this  mark  it  contains 
17.6  cubic  centimeters,  and  17.5  cubic  centimeters  or  18  grams  of 
milk  is  delivered  into  the  test  bottle,  .1  cubic  centimeter  adhering  to 
the  inner  surface  of  the  glass. 

The  tester. — The  centrifugal  machines  used  for  the  Babcock  test 
vary  in  size  from  2-bottle  to  48-bottle  machines.  These  machines  may 
be  operated  either  by  hand  or  by  steam,  or  electricity.  (Fig.  3.)  In 
all  cases,  it  is  essential  to  have  the  centrifuge  securely  fastened  to 
its  base. 


Circular  230]         TESTING    MILK   AND   CREAM    FOR   BUTTERFAT 


5 


Fig.  1. —  (1)  Type  of  rod  commonly  used  for  stirring  milk.  (2)  Dipper  type 
of  sampler  now  in  general  use  for  taking  milk  samples.  (3)  The  McKay  sampler 
occasionally  used  for  milk  sampling  but  designed  for  cream  sampling.  (4)  Thief 
sampler  which  is  efficiently  used  for  milk  sampling.  (5)  Two  types  of  bottles 
satisfactory  for  preserving  composite  samples. 


4  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

The  steam  machines  are  preferred  to  the  other  types  owing  to  the 
uniformity  of  speed  and  also  to  the  maintenance  of  an  even  tempera- 
ture which  they  make  possible. 

Acid. — Commercial  sulphuric  acid  with  a  specific  gravity  of  1.82- 
1.83  is  used  for  the  test.  It  is  essential  to  have  acid  of  this  strength 
in  order  to  obtain  clear  butter  fat  readings.  The  acid  should  always 
be  kept  in  a  tightly  closed  container  as  the  sulphuric  acid  weakens 
when  exposed  to  the  air,  owing  to  its  property  of  absorbing  moisture 


3 
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17.5 

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Fig.  2. —  (1)  Milk  test  bottles,  8  per  cent.  (2)  Milk  pipettes,  17.6  cubic 
centimeters  capacity.  (3)  Acid  measure,  17.5  cubic  centimeters  capacity.  (4) 
Acidometer. 


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from  the  atmosphere.  Sulphuric  acid  is  extremely  dangerous  owing 
to  its  corrosive  action.  When  acid  is  spilled,  it  is  essential  to  remove 
it  immediately  with  water,  and  then  check  the  corrosive  action  of  the 
acid  with  an  alkali.  Lead  sheeting  is  used  to  protect  testing  tables 
as  it  is  not  attacked  by  sulphuric  acid. 

The  acidometer  illustrated  in  Fig.  2  is  the  instrument  commonly 
used  for  determining  the  strength  of  commercial  sulphuric  acid.  It 
is  standardized  to  read  directly  the  specific  gravity  of  sulphuric  acid 
at  60°  F. 

Convenient  apparatus  for  use  in  disposing  of  waste  acid  is  shown 
in  Fig.  4. 


Circular  230]        TESTING  MILK  AND  CREAM  FOR  BUTTERFAT 


DIRECTIONS    FOR    MEASURING    MILK     INTO    THE    TEST    BOTTLE 

Pour  the  milk  sample  from  one  container  into  another  several  times 
to  insure  thorough  mixing.  Mixing  should  be  done  by  pouring  down 
the  side  of  the  container  and  not  into  the  center  of  it.  This  is  neces- 
sary so  as  to  avoid  the  introduction  of  air  into  the  milk.  Then  immedi- 
ately draw  the  milk  into  the  pipette  some  distance  above  the  mark. 
By  a  slight  release  of  the  forefinger  on  the  mouth  of  the  pipette  the 
milk  is  lowered  directly  to  the  mark.  The  stem  of  the  pipette  is  then 
inserted  into  the  neck  of  the  test  bottle  and  the  milk  allowed  to  run 
in.  By  blowing  into  the  pipette  the  last  drop  of  milk  is  forced  into 
the  bottle.  In  cases  in  which  the  results  of  testing  are  of  great  im- 
portance, as  in  the  testing  of  cows  for  advanced  registry,  all  tests 
should  be  made  in  duplicate.  The  milk  bottles  should  be  marked  so 
that  the  samples  can  be  identified. 


H 


Fig.  3. —  (1)  Steam  turbine  tester.  (2)  Electric  centrifuge.  (3)  Hand  centri- 
fuge. 

Adding  acid. — The  milk  and  acid  should  be  at  a  temperature  of 
from  55°-70°  P.,  at  the  time  the  acid  is  added  to  the  sample.  In 
adding  the  17.5  cubic  centimeters  of  acid  to  the  milk,  it  is  essential 
to  slope  the  test  bottle  so  the  acid  will  pass  down  the  side  of  the  neck 
of  the  bottle.  Adding  acid  in  the  central  region  of  the  bottle  would 
cause  a  charring  of  the  sample  and  might  cause  the  spilling  of  acid. 
The  acid  and  milk  should  be  mixed  by  a  rotary  motion  of  the  bottle. 

Centrifugmg  the  samples. — The  samples  should  be  placed  in  the 
tester  and  centrifuged  for  a  period  of  &\e  minutes,  If  all  the  pockets 
in  the  tester  are  not  filled  with  test  bottles,  the  bottles  should  be  evenly 
distributed  around  the  tester  so  that  the  centrifuge  will  be  evenly 
balanced.  The  speed  of  the  centrifuge  is  determined  by  the  diameter 
of  the  wheel.     (See  Table  1.) 

The  bottles  are  filled  to  the  base  of  the  neck  with  water  after  the 
five  minute  period.  To  insure  clear  tests,  the  water  used  should  be 
either  soft  or  distilled,  and  when  adding  it  to  the  tests  it  should  be 


b  UNIVERSITY    OF    CALIFORNIA — EXPERIMENT    STATION 

at  a  temperature  of  from  140°  to  180°  F.  Centrifuge  the  bottles  for 
two  minutes  after  this  addition  of  water.  Then  stop  the  tester  and 
add  water  up  to  the  7  per  cent  mark.  Centrifuge  one  minute  longer, 
and  remove  the  bottles  from  the  tester  for  reading. 

Reading  the  tests. — The  samples  are  placed  in  a  water  bath 
(Fig.  5)  at  a  temperature  of  130°  to  140°  F.  for  a  period  of  five 
minutes  or  more.  Care  should  be  taken  that  the  fat  in  the  neck  of 
the  bottle  is  completely  below  the  level  of  the  water  in  the  bath. 


Fig.  4. — Convenient  jar  for  handling  waste  acid. 


In  reading  the  fat  column  is  conveniently  measured  with  a  pair  of 
dividers.  The  measurement  should  include  the  lower  and  the  upper 
curve  of  the  fat  column.  (See  Fig.  7.)  Then  transfer  one  point  of 
the  divider  to  the  zero  mark  on  the  neck  of  the  bottle.  The  other 
divider  point  will  indicate  directly  the  per  cent  of  fat  in  the  sample. 


vm 


TABLE  I 

Showing  Speed  of  Centrifuge  for  Testers  of  Different  Diameters 


Diameter 

Revolutions 

Diameter 

Revolutions 

of  wheel 

of  wheel 

of  wheel 

of  Wheel 

in  inches 

per  minute 

in  inches 

per  minute 

10    

1000-1100 

18   

775-  825 

12  

900-1000 

20  

725-  775 

14  

825-  975 

22  

700-  750 

16  

825-  675 

24  

575-  625 

CIRCULAR  230]  TESTING    MILK   AND    CREAM    FOR   BUTTERFAT  7 

Abnormal  tests. — Charred  samples  may  result  from  using  too  much 
acid,  using  warm  acid  or  milk,  adding  acid  directly  to  the  milk  instead 
of  permitting  it  to  run  down  the  side  of  the  neck  by  sloping  the  bottle, 
allowing  samples  to  remain  too  long  before  centrifuging,  or  from 
using  acid  of  a  strength  greater  than  that  specified,  or  allowing  milk 
and  acid  to  stand  before  mixing.  The  formation  of  curd  in  the  tests 
is  due  either  to  using  weak  acid,  insufficient  mixing,  or  to  using  milk 
or  acid  that  is  too  cold.  Using  hard  water  causes  the  formation  of  a 
white  foam. 


Fig.  5. — Type  of  water  bath  used  for  milk  and  cream  samples. 


TESTING  CREAM  FOR  BUTTER  FAT 

The  range  in  specific  gravity  of  cream  of  various  fat  contents  is 
very  great.  Hence,  an  18  gram  sample  of  cream  cannot  be  obtained 
by  means  of  a  17.6  cubic  centimeter  measurement.  Furthermore,  as 
cream  of  high  test  has  a  thick  consistency,  much  of  it  would  adhere 
to  the  walls  of  the  pipette  in  case  it  were  measured  into  the  test  bottle. 
Since  separator  cream  also  contains  more  or  less  air  it  is  essential  to 
weigh  it  in  order  to  obtain  a  proper  sample. 

Cream  test  bottles. — The  bottles  used  for  testing  cream  by  the  Bab- 
cock  method  are  marked  with  divisions  on  the  neck  of  the  bottles  to 
read  from  zero  to  50  per  cent.  Eighteen  gram  or  9  gram  bottles 
are  used.    They  are  designed  to  read  directly  the  percentage  of  butter 


s 


UNIVERSITY    OF    CALIFORNIA — EXPERIMENT    STATION 


fat.  The  smaller  divisions  on  the  necks  of  both  types  of  bottles  repre- 
sent five-tenths  per  cent  (.5%)  each.  The  types  of  cream  bottles 
commonly  used  are  shown  in  Fig.  6. 

Cream  test  balances. — The  balances  used  in  weighing  cream  are 
specially  designed  and  range  in  capacity  form  one  to  twelve  bottles. 
The  four-bottle  Torsion  Balance  illustrated  in  Fig.  6  is  very  satis- 


Fig'.  6. — Cream  balance  and  test   bottles. 


factory.  Cream  balances  should  be  carefully  checked  for  accuracy 
by  a  person  familiar  with  their  construction.  Careful  handling  of 
the  cream  balance  is  imperative  for  good  results. 

Preparation  of  the  cream  sample  for  testing. — A  sample  of  cream 
can  be  prepared  by  pouring  it  from  one  container  to  another  in  a 
manner  similar  to  that  described  for  milk.  Most  cream  samples,  how- 
ever, are  clotted  when  they  are  presented  to  the  operator  for  analysis. 
These  samples  are  placed  in  a  bath  of  water  at  90°  to  110°  F.  This 
treatment  restores  the  liquid  condition  of  the  cream  and  makes  it 
possible  to  follow  the  procedure  described  for  preparing  milk  samples. 


Circular  230]         TESTING   MILK   AND    CREAM   FOR   BUTTERFAT  9 

Weighing  the  cream  for  analysis. — The  bottles  are  placed  in  the 
sockets  of  the  balance  pan  and  balanced.  By  means  of  a  pipette,  a 
charge  of  cream  is  passed  into  the  cream  test  bottle.  Considerable 
skill  is  required  by  the  operator  in  order  to  check  the  flow  of  cream 
when  the  pointer  of  the  balance  indicates  that  the  correct  charge 
has  been  delivered.  If  too  much  cream  is  added,  a  small  portion  can 
be  poured  from  the  test  bottle  and  the  bottle  again  placed  upon  the 
scales  and  cream  added  from  the  pipette  until  it  is  accurately  balanced. 

Adding  the  acid. — Cream  contains  less  solids-not-fat  than  milk  and 
for  this  reason  less  acid  is  required  in  making  the  cream  test.  The 
amount  of  acid  necessary  to  be  used  will  decrease  as  the  richness  of 
the  cream  increases.  The  development  of  a  coffee  brown  color  after 
mixing  the  acid  and  cream  is  an  indication  that  sufficient  acid  has 
been  added.  The  development  of  a  darker  color  is  a  sign  of  too  much 
acid.  Under  normal  conditions,  with  the  cream  and  acid  at  a  tem- 
perature of  70°  F.,  approximately  4  to  8  cubic  centimeters  of  acid 
are  required  for  a  9  gram  sample,  and  8  to  12  cubic  centimeters  would 
be  required  for  an  18  gram  sample  of  average  cream. 

While  adding  the  acid,  it  is  necessary  to  revolve  the  cream  bottle 
so  as  to  allow  the  acid  to  wash  the  cream  into  the  bulb  of  the  bottle. 
In  mixing,  the  bottle  should  be  rotated  evenly  so  that  none  of  the 
mixture  is  expelled  from  the  bottle. 

After  mixing  the  cream  and  acid,  water  at  a  temperature  of  about 
180°  F.  is  added  to  the  sample.  The  addition  of  water  should  be 
sufficient  to  bring  the  contents  of  the  bottle  up  to  the  base  of  the  neck. 
The  purpose  of  adding  the  water  before  whirling  is  to  check  the  action 
of  the  acid  and  prevent  charring. 

Centrifuging  the  samples. — The  initial  whirling  should  cover  a 
period  of  five  minutes  as  for  milk.  The  fat  is  then  raised  into  the 
neck  of  the  bottle  by  the  addition  of  water  at  a  temperature  of  about 
180°  F.  After  this,  the  bottles  receive  a  final  whirling  for  a  period  of 
three  minutes. 

Beading  the  tests. — At  the  completion  of  the  second  whirling,  the 
bottles  are  placed  in  a  bath  of  water  at  135°  to  140°  F.,  for  a  period 
of  at  least  ten  minutes.  Just  before  reading  the  tests,  a  few  drops  of 
glymol  (one  quart  of  a  white  mineral  oil  called  Hydrol  colored  with  1 
oz.  alkanet  root)  are  added  to  the  tests.  The  addition  of  the  glymol 
straightens  the  upper  meniscus  or  crescent-shaped  curve  of  the  fat, 
making  possible  an  easier  and  more  accurate  reading.     The  tests  are 


10 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


then  read  by  the  aid  of  a  pair  of  dividers.  The  reading  is  taken  from 
the  lower  part  of  the  lower  meniscus  to  the  straight  line  formed  by 
the  upper  meniscus  and  the  glymol.     (Fig.  7.) 


THE  PROPER.  METHOD 
CREfiM  3  MILK  TEST5 


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145 

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26% 


TOM1UC 

TE5I5 
4% 


PROPER  METHOD  ^READI/IG-  TE5T5    FROM    ft.  TO  B . 
IMPROPER  METHOD^READKIG-TE^rS  FROM   fi.TOC. 


Fig.   7. — Chart  showing  proper   method   of   reading  milk  and   cream  tests. 

Abnormal  tests  are  attributed  to  practically  the  same  causes  de- 
scribed for  abnormal  milk  tests.  The  readings  of  a  fat  column  that 
appears  abnormal  should  never  be  recorded. 


Circular  230]         TESTING   MILK   AND   CREAM   FOR  BUTTERFAT  11 


TESTING    SKIM-MILK,    BUTTERMILK    AND    WHEY    FOR    BUTTER    FAT 

Owing  to  the  small  amount  of  fat  contained  in  skim-milk,  butter- 
milk, and  whey,  a  bottle  of  special  construction  is  required  for  testing 
them  by  the  Babcock  method.  The  bottles  known  as  double-neck  skim- 
milk  bottles  have  one  large  funnel  neck  for  the  charge.  The  other 
neck  has  a  fine  bore  and  has  graduations  sufficiently  small  to  record 
.01  per  cent  fat.  The  neck  is  graduated  to  record  a  fat  content  of 
.50  per  cent. 

In  testing  these  products  the  directions  outlined  for  milk  testing 
are  used.  The  mixing  is  equally  as  important  as  in  milk  or  in  cream 
testing. 

The  17.5  cubic  centimeter  charge  is  delivered  into  the  funnel-like 
neck.  In  whirling  it  is  necessary  to  place  the  bottles  in  the  tester  so 
that  the  funnel  neck  faces  the  center  of  the  centrifuge.  The  double 
necked  bottles  are  of  delicate  construction  and  care  should  be  taken 
in  centrifuging  to  maintain  an  even  speed  so  as  to  avoid  breakage. 

The  percentage  of  solids-other-than-fat  in  the  serum  of  skim-milk, 
buttermilk,  and  whey  vary  widely,  hence,  the  same  rule  in  regard  to 
the  amount  of  acid  to  use  is  the  same  as  in  the  testing  of  cream.  The 
amount  of  acid  required  for  complete  solution  of  the  solids-not-fat  is 
determined  by  the  development  of  a  coffee  brown  color  in  the  sample, 
following  the  mixing  of  the  acid  with  the  sample.  For  normal  skim- 
milk  and  buttermilk  the  amount  of  acid  required  for  successful  oper- 
ation of  the  test  is  about  the  same  as  required  for  whole  milk. 

Since  whey  contains  a  smaller  amount  of  solids  than  skim-milk 
and  buttermilk,  a  smaller  amount  of  acid  is  required.  Ordinarily 
12  cubic  centimeters  of  sulphuric  acid  or  less  is  used  for  an  18  gram 
sample  of  whey. 

Whey,  buttermilk,  and  skim-milk  contain  a  certain  amount  of 
residual  fat  which  will  not  separate  from  the  serum  upon  centrifuging. 
Nevertheless,  accurate  testing  of  these  products  will  indicate  whether 
or  not  efficient  results  are  being  obtained  in  separating,  churning,  or 
in  cheese  making. 


